A natural shoulder joint may undergo degenerative changes caused by a variety of reasons. When these degenerative changes become so far advanced and irreversible, it may ultimately become necessary to replace a natural shoulder joint with a prosthetic shoulder joint. In the traditional implantation of a shoulder joint prosthesis, the natural head portion of the humerus is resected and a cavity is created in the intramedullary canal of the host humerus for accepting a humeral component. The humeral component generally includes a stem, and a head portion, which is used to replace the natural head of the humerus. Once the humeral component has been implanted, the glenoid cavity positioned at the scapula may also be resected and shaped to accept a glenoid component. The glenoid component generally includes an articulating surface or cup which is engaged by the head portion of the humeral component. Modular designs for the humeral and glenoid components are currently available for the traditional shoulder arthroplasty, and components of different sizes or shapes are at the disposal of the surgeon performing the operation.
The traditional shoulder joint implantation requires that the rotator cuff muscles be present and functioning normally. When this is not the case, a reverse shoulder prosthesis may be preferable. In the reverse shoulder prosthesis, the humeral component includes a stem, and a cup connected to the stem. The glenoid component supports a head which articulates with the cup.
It is not always practical to determine well in advance of the procedure whether a reverse or traditional shoulder prosthesis should be used. It is, therefore, desirable to provide a selection of modular components that can be combined for use in traditional as well as reverse shoulder arthroplasty, with the goals of increasing flexibility and choice and for providing interchangeable and easy to use components that are also cost effective.
When the glenoid cavity is intact, which is the case in the event of a comminuted fracture, shoulder prosthesis includes a humeral element fixed into the medullary cavity of the humerus provided with a male implant having spherical shape able to cooperate directly with the glenoid cavity.
In the case of destruction of the glenoid cavity, the prosthesis consists in an support implanted in the glenoid and provided with an insert male or female intended to cooperate with a corresponding shape arranged at the humeral element. Different kinds of prosthesis are known in the art.
FR2652498 discloses a modular humeral implant comprising an anchoring stem, provided at its upper end with a head having a recess arranged to receive a male or a female insert.
One disadvantage of this anchoring stem is that it requires, on the basis of the dimensions of the head of said stem, a significant resection of cancellous bone.
In addition, the positioning of the stem leads to modify the original positioning of the humeral head and of its center of rotation, which could lead to a fracture of the humerus metaphyseal area.
In general, the problems related to humeral component with a stems are the following: the offset between the centre of rotation of the head and the axis of the diaphysis, implantation surgery is more difficult and the required time is longer compared to stemless procedures, postoperative complications are dominated by fractures, the removal of the stem in case of revision surgery is necessary.
Different solutions have been proposed to avoid these complications: the use of modular prosthesis and the humeral head resurfacing. In an effort to reduce stem complications and to avoid the loosening of the humeral component, stemless prosthesis with methaphysial fixation are being developed such as that illustrated in EP1520560. The goal of the stemless prosthesis is to restore the anatomy of the humeral head without need of a stem, with automatic centering, through a simple and reproducible technique, with the preservation of the bone stock and an adequate exposure of the glenoid. The indications for a stemless prosthesis humeral implant are the same as the indications for traditional hemiarthroplasty, total shoulder replacement, or even humeral head resurfacing. The stemless prosthesis could be implanted in cases of significant bone loss, for example in advanced avascular necrosis. The main indication for stemless implantation remains painful gleno-humeral ostheoartris.
The indications for placement of a glenoid component are identical as in traditional total shoulder arthoplasty. The controindications on the stemless use are osteopenia, acute proximal humeral fractures and active infections.
EP1520560 discloses an anatomic humeral prosthesis consisting of a removable support for metaphyseal fixation and a humeral head, a male or a female insert able to cooperate with glenoid cavity, fixed to the removable support through a intermediate neck, i.e. a Morse taper. It further comprises an anchoring stem. The removable support has the shape of a basket provided with a circular seat with a through-opening and comprising at least 3 identical branches substantially in the shape of an arc or a circle, which are spaced apart at intervals and give said support a general hemispherical shape.
The prosthesis described in EP1520560 need to be removed from the patient in case of conversion from anatomical configuration to reverse.